INTRODUCTION: Obesity and overweight strongly contribute to increasing cardiovascular morbidity and mortality, and are becoming a worldwide health issue. The prevalence of obesity has increased dramatically in Latin America. Child obesity is a major issue. Fast food is strongly suspected of contributing to this epidemic of obesity, although there is a lack of evidence. METHODS: We studied the correlation between the number of McDonald restaurants and overweight and obesity prevalence by region stratified by gender and age. Data on prevalences were obtained within national studies conducted by the Brazilian Ministry of Health. Three age sub-groups were analyzed: 5 to 9-year-olds, 10 to 19-year-olds and over 19-year-olds. RESULTS: There was a very strong positive correlation between overweight rates and the number of McDonald restaurants for both males and females between 5 and 9 years old (R2 respectively = 0.92 and 0.84) and a strong positive correlation for females between 10 and 19 years old (R2 = 0.68). There was a very strong positive correlation between obesity rates and the number of McDonald restaurants for males between 5 and 9 years old (R2 = 0.95). This positive correlation was strong for both males and females between 10 and 19 years old (R2 respectively = 0.77 and 0.63). Other correlations were not significant. CONCLUSION: A strong correlation between the prevalence of overweight and obesity and the number of McDonald restaurants was found for Brazilian children and was most important within the group of youngest children. These results should be taken into consideration by education and prevention campaigns.